Sams Teach Yourself Transact-SQL in 21 Days, Second Edition

Grouping Data

So far, you've learned how to use aggregates to get overall totals (and averages, and so on) for a set of rows. If you wanted to know the total dollar sales for product 14, you could execute the query we wrote earlier:

select sum(Quantity * UnitPrice * (1.0 – Discount))
                     as 'Total Dollar Sales'
  from [Order Details]
 where ProductID = 14

What if you were also interested in products 11, 12, and 13? You could reexecute the query several times, each time with a different product ID.

It would be better to write a single query and get a result set that looked like this:

 ProductID Total Dollar Sales ----------- --------------------------------------- 11 12901.770042419434 12 12257.660041809082 13 4960.4400224685669 14 7991.4900035858154 ...